In Japanese Patent No. 4032313 (Patent Literature 1), there is described that a projection bolt including a shank, an enlarged diameter portion formed integrally with the shank, and a welding projection arranged at a center of the enlarged diameter portion is welded to a steel sheet component by electric resistance welding.
The projection bolt disclosed in Patent Literature 1 has a shape illustrated in FIG. 10A. A projection bolt 20 of FIG. 10A is made of iron, and includes a shank 21 having an external thread formed therein, a circular enlarged diameter portion 22 formed integrally with the shank 21 and having a diameter larger than a diameter of the shank 21, and a circular welding projection 23 arranged at a center of the enlarged diameter portion on a side opposite to the shank 21. The welding projection 23 is a circular bulged portion having a diameter smaller than the diameter of the enlarged diameter portion 22, and includes a tapered portion 24 formed on a distal end surface side thereof at a small inclination angle, and a vertex 25 having a pointed center portion. An end surface of the enlarged diameter portion 22 excluding the welding projection 23 is formed into a tapered surface 26 reduced in height as approaching to an outer peripheral side thereof.